A fuel cell generates electricity directly through electrochemical reactions and is more efficient than a heat engine because it eliminates the mechanical or rotating machinery. The electrical energy conversion efficiency of most fuel cells is relatively high, ranging from 40 to 60 percent based on the lower heating value (LHV) of the fuel, regardless of size and load.
Because of their inherent efficiency and potential for low emissions, high-temperature fuel cells (HTFC) such as solid oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC) have been the subject of intense research for several decades.
Fuel cells are associated with some significant drawbacks despite their high energy conversion efficiency.
Firstly, the remaining energy content of a fuel cell system that is not converted to electricity, a high level on the order of 50%, is lost as waste heat through the fuel cell cooling system and exhaust gas heat. The energy efficiency of a power producing system would increase significantly if a portion of the fuel cell waste heat would be able to be exploited.
Secondly, fuel cells requiring high capital costs have a relatively short life span, generally ranging from two to five years, due to the high temperature and accepted electrolyte type that lead to corrosion of the anode and cathode. Thus, the efficiency of a MCFC and SOFC decreases over time, their power output decreases, and the exhaust heat from these fuel cells increases. As a result of such fuel cell degradation and poor profitability, their commercial life span is limited. Additional expenditures are required if it is desired to recycle a fuel cell. The use of fuel cells for power generation has therefore been quite limited up to the present day.
There have been several attempts in the prior art to generate power from degraded fuel cells including: (a) using an array of fuel cells each having a low power output rating, and deactivating one of them during periods of reduced load, (b) using a parasitic load stack to regulate the power, and (c) controlling the flow of fuel and oxygen/air through the fuel cell.
It is an object of the present invention to provide a hybrid fuel cell based power plant that is able to profitably employ fuel cells for a period of time generally beyond their normal life span.
It is an additional object of the present invention to further increase the power output and efficiency of a hybrid fuel cell based power plant.
It is an additional object of the present invention to efficiently utilize the waste heat of a fuel cell for generating additional power in a power plant.
Other objects and advantages of the invention will become apparent as the description proceeds.